1. Field of the Invention
The present invention relates to an image fixing roller for thermally fixing images on an image receiving material, an image fixing apparatus comprising the image fixing roller, and a method of fixing toner images on an image receiving material, using the image fixing roller, which are used in image formation apparatus, such as a copying machine, printer and facsimile apparatus.
2. Discussion of Background
In conventional image formation apparatus such as a copying machine, printer and facsimile apparatus, developed toner images are fixed on an image receiving material by use of an image fixing apparatus comprising an image fixing roller and a pressure application roller.
In the image fixing apparatus, the image receiving material to which developed toner images are transferred is caused to pass between the image fixing roller and the pressure application roller, and the toner of the developed toner images is fused or softened and then thermally fixed to the image receiving material.
This kind of image fixing roller is warmed up before use until the outer peripheral surface of the image fixing roller reaches a predetermined temperature which is necessary for image fixing, that is, an image-fixing possible temperature, for instance, to 180.degree. C. Since this warm-up takes a relatively long period of time, a preheating system for starting the preheating of the image fixing roller when a main switch of the image formation apparatus is turned on is in general use.
However, the power consumption of the preheating system for the image fixing roller is so large that this kind of preheating is not always preferable for use in view of global environment conservation and energy saving.
The applicants of the present application previously proposed an image fixing roller comprising a cylindrical core metal, an exothermic phase transition layer provided on the cylindrical core metal, comprising an exothermic phase transition material capable of performing reversible phase transition from an amorphous state to a crystalline state and vice versa, and a protective layer provided on the exothermic phase transition layer, as disclosed, for example, in Japanese Laid-Open Patent Application 7-140823. When this image fixing roller is used, before the outer peripheral surface of the image fixing roller is caused to reach the image-fixing possible temperature by a heater, the temperature elevation rate of the outer peripheral surface of the image fixing roller is significantly increased by the thermal energy which is released when the phase transition of the exothermic phase transition material from an amorphous state to a crystalline state is carried out, in comparison with the temperature elevation rate of the outer peripheral surface of the image fixing roller which is attained only by use of the heater, whereby the shortening of the warm-up time for the image fixing roller and the power consumption therefor are attained.
In this image fixing roller, since the thermal energy which is liberated when the exothermic phase transition material is crystallized is utilized, it is necessary that the exothermic phase transition material be rapidly cooled to change its state from a fused state to an amorphous state and have the properties that the exothermic phase transition material in the amorphous state can be changed to a crystallized state when the temperature of the exothermic phase transition material is elevated.
Examples of inorganic exothermic phase transition materials that can be used as the above-mentioned exothermic phase transition material are multi-element materials composed of any of elements of Group III through Group IV of the Periodic Table which are known as having a region of becoming amorphous. Of such inorganic exothermic phase transition materials, chalcogen and chalcogenide compounds can be rapidly crystallized to liberate a large quantity of crystallization heat and therefore are particularly preferable exothermic phase transition materials for use in the above-mentioned image fixing roller.
Furthermore, as organic exothermic phase transition materials that can be used as the above-mentioned exothermic phase transition material, crystalline thermoplastic resins, for example, polyesters such as PET (polyethylene terephthalate) and PBT (polypropylene terephthalate) resins, are known as having a region of becoming amorphous. Furthermore, it is known that low-molecular weight organic materials such as diphenyl isopthalate derivatives and bisphenol derivatives exothermically liberate heat when crystallized.
For example, FIG. 1 is a graph showing the differential thermal analysis characteristics of a representative exothermic phase transition material (Se) measured by a differential thermal analyzer (DTA). In FIG. 1, L1 indicates a control temperature straight line. FIG. 1 shows an exothermic-endothermic curve Q of the exothermic phase transition material (Se) at the time of a 10-degree temperature elevation per 10 minutes. Tg indicates the glass transition temperature of the exothermic phase transition material (Se); Pg and Pm, the endothermic peaks thereof; Pc, an exothermic peak of thereof; Tcp, the exothermic peak temperature thereof; Tci, the crystallization initiation temperature thereof at which the phase transition from an amorphous state to a crystalline state is initiated; Tcf, the crystallization finalization temperature thereof at which the phase transition of the material (Se) is finalized and the material (Se) reaches the control temperature; and Tm, the fused temperature thereof or the melting point thereof. These temperature characteristics of the material (Se) slightly shift to a higher temperature side as the control rate is increased.
With reference to this exothermic-endothermic curve Q, the small endothermic peak Pg is first observed at the glass transition temperature Tg in the course of the passage of time or the elevation of the temperature, and the large exothermic peak Pc is then observed, which is caused to appear by the crystallization of the material (Se). Subsequently, the endothermic peak Pm is then observed, which is caused to appear by the melting of the material (Se).
In order to further shorten the warm-up time of the image fixing roller, it is necessary that the temperature of the outer peripheral surface of the image fixing roller be quickly elevated to a temperature above the image fixing possible temperature or the toner softening or fusing temperature.
If the exothermic phase transition material is caused to exothermically liberate heat at a temperature level which is far below the image fixing possible temperature, the exothermically liberated heat is caused to dissipate away before the temperature of the outer peripheral surface of the image fixing roller reaches the image fixing possible temperature, so that the exothermic phase transition material cannot be used effectively for shortening the warm-up time of the image fixing roller.
On the other hand, if the exothermic phase transition material is caused to exothermically liberate heat after the outer peripheral surface of the image fixing roller reaches the image fixing possible temperature, the warm-up time of the image fixing roller cannot be shortened.
If the exothermic temperature range in which the exothermic material liberates heat and terminates the liberation of the heat is excessively higher than the image fixing possible temperature, the liberated heat increases the temperature of the surface of the image fixing roller even after the surface of the image fixing roller reaches the image fixing possible temperature, so that the so-called overheating of the image fixing roller takes place.
However, each exothermic phase transition material has its own particular crystallization temperature characteristics such as crystallization initiation temperature Tci, exothermic peak temperature Tcp, melting point Tm, and crystallization finalization temperature Tcf, so that it is desired to obtain an exothermic phase transition material having suitable crystallization temperature characteristics for the image fixing roller, for instance, an exothermic phase transition material with the temperature range from the crystallization initiation temperature Tc through the melting point Tm thereof being in the range of 80 to 200.degree. C. for use with a commercially available image fixing roller. However it is extremely difficult to obtain an exothermic phase transition material with the above-mentioned temperature range.